Host immune defense against bacterial infection requires activation of conserved signaling pathways that upregulate production of inflammatory mediators to clear infection. Many bacterial pathogens inhibit these signaling pathways in order to evade host immune defenses. Such pathogens include the pathogenic Yersiniae which utilize the YopJ virulence factor to block NF-?B and MAPK signaling. How immune defense is mediated against pathogens that block immune signaling pathways remains poorly understood. NF-?B blockade in infected macrophages leads to cell death with characteristics of apoptosis, pyroptosis, and necrosis. Surprisingly, increasing cytotoxicity of Y. pseudotuberculosis or Y. pestis results in dramatically decreased virulence of the bacteria, suggesting that induction of cell death in response to Yersinia infection may be a host defense mechanism. Our central hypothesis is that cell death triggered in response to Yersinia blockade of NF- ?B and MAPK leads to pro-inflammatory cell death that alerts uninfected neighboring cells to the presence of infection. However, the mechanisms of YopJ-induced cell death and the precise role of cell death in immune defense against infection remain unclear. Multiple caspases are activated during Yersinia infection, but no specific factor(s) are yet known that are required for Yersinia-induced cell death. Our preliminary data demonstrate that caspsase-8 is required for Yersinia-induced cell death, and for host defense in response to Yersinia infection in vivo. Furthermore, caspase-8 is required for caspase-1 processing in response to Yersinia, but not in response to other caspase-1 activating stimuli. Caspase-8-dependent caspase-1 activation is a novel pathway that enables an apoptotic caspase to activate a pyroptotic cell death. However, the molecular mechanism of caspase-8-dependent caspase-1 activation remains unknown. This is an important problem as it likely functions in response to many pathogens that block critical innate immune signaling pathways and in the context of pathological stimuli that lead to caspase-8 activation We propose two Specific Aims to address this important gap in our knowledge. First we will define the molecular basis of caspase-8-dependent activation of caspase-1, and dissect the relative contribution of distinct cell death pathways to Yersinia-induced cell death. Second, we will define the role of caspase-8 in immune defense against Yersinia infection.